The disclosure relates to display devices.
Electroluminescence (EL) display devices include organic light emitting diode (OLED) displays and polymeric light emitting diode (PLED) displays. In accordance with associated driving methods, an OLED can be an active matrix type or a positive matrix type. An active matrix OLED (AM-OLED) display typically is thin and exhibits lightweight characteristics, spontaneous luminescence with high luminance efficiency and low driving voltage. Additionally, an AM-OLED display provides the perceived advantages of increased viewing angle, high contrast, high-response speed, full color and flexibility.
An AM-OLED display is driven by electric current. Specifically, each of the matrix-array pixel areas of an AM-OLED display includes at least one thin film transistor (TFT), serving as a driving TFT, to modulate the driving current. Driving current is modulated based on the variation of capacitor storage potential to control the brightness and gray level of the pixel areas.
The gray level is selected by using a voltage divider comprising resistors. FIG. 1a is a schematic diagram of a conventional voltage divider. The voltage divider 10 comprises resistors serially connected between a high voltage source (Vcc) and a low voltage source (Gnd). Each point between two resistors has a corresponding voltage indicating a particular gray level.
A point 110 of voltage divider 10 can provide a maximum gray level indicating a maximum brightness of the AM-OLED. Since a voltage divider only provides one maximum gray level, if a user desires to adjust the maximum brightness of the AM-OLED higher, the AM-OLED requires several voltage dividers.
FIG. 1b is a schematic diagram of another conventional voltage divider. A voltage between two resistors can be adjusted according to the resistance of two resistors. In this case, a first maximum gray level provided by voltage divider 10 is 100 nits, a second maximum gray level provided by voltage divider 12 is 150 nits, and a third maximum gray level provided by voltage divider 14 is 200 nits. Therefore, the brightness of the AM-OLED can be adjusted by providing different maximum gray levels; however, the cost and volume of the AM-OLED are increased.